if a pregnant lady craves for something not kosher is she permitted to eat it?
This depends on the level of the craving, and on whether there is a kosher alternative.
Based on the laws of Yom Kippur, if a pregnant lady tells us of her craving for a food that is not kosher (or if “her face changes” out of craving), and there is no kosher alternative that would satiate her, then she should be told that the food is not kosher. If this does not help, then she should be given a small amount of the food — less than a shiur. If this still doesn’t help, then she is permitted to eat it.
Sources: See Shulchan Aruch 617:1-2; Mishnah Berurah 617:1, 5; Shulchan Aruch Harav 617:2.